Chalk is a sedimentary rock consisting mostly of micrometer sized particles of biogenic calcite (CaCO 3). It forms the reservoirs for oil in the Danish part of the North Sea and important drinking water aquifers in northern Europe. The interaction between organic molecules and the particle surfaces has a strong influence on the migration of oil in the reservoirs. Adhesion of oil components to chalk particle surfaces is thought to significantly reduce production and efforts are being made to determine the mechanisms behind enhanced/improved oil recovery (EOR/IOR) during water flooding. Aquifers beneath industrial areas can be heavily polluted with organic compounds and similar adsorption mechanisms could influence the geographical extent of the contaminant plume. Chalk, as a sedimentary rock, has been studied extensively for many years but the properties of the particle surfaces, where adsorption takes place, are largely unknown. In this study, we have used atomic force microscopy (AFM) to show that the grain surfaces in offshore and onshore chalk are more heterogeneous than previously assumed. The particles are not simply calcite surfaces but are partially covered by clay that is only 1-4nm thick. With chemical force mapping (CFM), we have probed the surface using a tip coated with organic molecules to represent a tiny, mono-functionalized oil droplet. The functional groups of the self assembled monolayer on the tip were either -CH 3 or -COO -. From maps of adhesion, it is evident that in calcite saturated water, both the polar and the nonpolar functional groups adhere to the nano sized clay particles but not to calcite. This is fundamentally important information for the development of conceptual and chemical models to explain wettability alterations in chalk reservoirs